1. Field of the Invention
The present invention relates to a sublimation-type thermal image transfer recording medium, and more particularly to a sublimation-type thermal image transfer recording medium for the multiple printing method, which comprises a dye-supply layer, a dye-transfer layer and a top layer which are successively overlaid on a support.
2. Discussion of Background
Recently the demand for full color printers is increasing year by year. Typical recording methods for full color printers now available include the electrophotographic method, the ink-jet method, and the thermosensitive image transfer method. Of these methods, the thermosensitive image transfer method is most widely employed because of its advantages over the other methods in that the maintenance is easy and the operation is noiseless.
In the thermosensitive image transfer recording method, a thermal image transfer recording medium, that is, a color ink sheet, and an image-receiving sheet are employed. The above-mentioned thermal image transfer recording medium comprises a support and an ink layer formed thereon in which a coloring agent is dispersed in a thermofusible material, or a sublimable dye is dispersed in a binder resin.
To carry out the thermosensitive image transfer recording, the image-receiving sheet is superimposed on the ink layer of the thermal image transfer recording medium and a color ink is transferred imagewise from the thermal image transfer recording medium to the image-receiving sheet by the thermal fusion of the coloring agent or the sublimation of the sublimable dye under the application of thermal energy to the recording medium by laser beams or through a thermal head which is energized by the electric signals corresponding to the images to be recorded.
The thermosensitive image transfer recording methods can be roughly classified into two types, a thermal fusing image transfer type and a sublimation image transfer type. The sublimation image transfer type is advantageous over the thermal fusing type in that a halftone can be obtained without difficulty and the image gradation ca be controlled as desired. These advantages can be obtained because a sublimable dye is in principle sublimated in the form of individually separated molecules in such an amount as to correspond to the amount of thermal energy applied thereto, for instance, from a thermal head, and transferred to the image-receiving sheet. Therefore, the sublimation image transfer type is considered to be most suitable for color printers.
The sublimation image transfer recording method, however, has a shortcoming in that its running cost is high, because in this image transfer method, a yellow ink sheet, a magenta ink sheet, a cyan ink sheet and, if necessary, a black ink sheet, are employed in order to obtain a full-color image, with selective application of thermal energy to each ink sheet, and discarded after the recording even though large unused portions remain on each ink sheet.
To eliminate this shortcoming, a multiple printing recording method has been proposed, by which an ink sheet can used repeatedly.
The multiple printing recording method can be accomplished by either an equal-speed mode method or an n-times-speed mode method. In the former method, an ink sheet and an image-receiving sheet are moved at the same speed when images are recorded. In the latter method, the running speed of the image-receiving sheet is n (n&gt;1) times the running speed of the ink sheet when images are printed, so that the ink sheet is shifted relative to the image-receiving sheet in such a manner that a preceding portion of the ink sheet and the following portion thereof partly overlap with respect to the ink transfer therefrom in the course of the thermal printing. Therefore as a matter of course, the larger the value of "n", the larger the cost reduction in printing.
In such an n-times-speed mode method, the ink is supplied at least from a newly used portion of the ink sheet in each printing, so that the amount of a residual ink in the ink sheet can be maintained constant in comparison with the equal-speed mode method in which a used portion of the ink sheet is merely used repeatedly. Therefore, the n-times-speed mode method is advantageous over the equal-speed mode method with respect to the minimization of the amount of the residual ink in the ink sheet from the viewpoint of the recording history of the ink sheet as reported in the Journal of the Institute of Electronics and Communication Engineers, Vol. J70-C, No. 11, pages 1537-1544 (1987).
The multiple printing recording methods such as the n-times-speed mode method and the equal-speed mode method, however, have the drawback that the sublimable dye which has been transferred to the image-receiving sheet is transferred back to the ink layer of the thermal image transfer recording medium. As a result, the color of the image subsequently formed becomes turbid and a ghost image is formed on the image-receiving sheet, so that clear images cannot be obtained.
The previously mentioned "ghost image" is formed in such a fashion that a reversely transferred image from the image-receiving sheet to the thermal image transfer recording medium is transferred again to a different portion of the image-receiving sheet in the equal-speed mode method.
In the n-times-speed mode method, since the thermal image transfer recording sheet is continuously moved, an edge portion of the obtained image tends to become tail-shaped.
The above-mentioned drawback in the sublimation image transfer recording method stems from the thermal diffusion of the sublimable dye from the ink layer of the thermal image transfer recording medium to the image-receiving sheet, which are closely brought into pressure contact with each other by a thermal head and a platen roller.
When a secondary or tertiary color is formed on the image-receiving sheet by superimposing two or three dyes in the full-color printing process, the dye which has been already transferred to the image-receiving sheet is transferred back to the ink layer of the thermal image transfer recording medium. This phenomenon is hereinafter referred to as "reverse transfer." In the one-time printing method, since the thermal image transfer recording sheet in which the reverse transfer has occurred is discarded, the above-mentioned problem does not affect the image formation. In the multiple printing recording method, however, the image reversely transferred to the ink layer of the thermal image transfer recording sheet is transferred again to the other position of the image-receiving sheet, and causes the color turbidity and forms a ghost image, thereby adversely affecting the subsequent recording. This adverse effect can be visually reduced to some degree by changing the order of colors to be transferred. However, this method cannot satisfactorily solve the problem.
To solve the reverse transfer problem, there is disclosed in Japanese Laid-Open Patent Application 61-293891 a method of forming each color image on a different transparent image-receiving sheet and overlapping the image-receiving sheets to obtain a full-color image. This method, however, has the shortcomings that such special image-receiving sheets are necessary, the thus prepared image-receiving medium is thicker than conventional image receiving media since three image-receiving sheets are used, and costly. Furthermore, this method has the risk that the obtained full-color image is made blurred even by a slight deviation of the registration of the three image-receiving sheets.